1. Field of the Invention
This invention relates to a scanning exposure method which sequentially exposes a mask pattern on a photo-sensitive substrate with, for example, a slit scanning or a step-and-scan type apparatus, and, more particularly, to one suitable when applied to a case where exposure is performed by a scanning exposure in autofocusing and autoleveling.
2. Related Prior Arts
In manufacturing a semiconductor device, a liquid crystal display element, a charge-coupled device (CCD), or a thin film magnetic disk with a photolithography technique, a projection exposure apparatus has been used which transfers a pattern on a reticle (or photomask, etc.) onto a wafer (or glass plate, etc.) to which a photosensitive agent is applied. A conventional projection exposure apparatus frequently used is a reduction projection exposure apparatus of a step-and-repeat type (stepper) which sequentially moves each shot area on a wafer to an exposure field of an optical projection system, and exposes reticle pattern images onto each shot area in a ground.
Such a stepper projection exposure apparatus is provided with an autofocusing mechanism and an autoleveling mechanism, both of which align each shot area on the wafer with the imaging plane of the optical projection system. These autofocusing and autoleveling mechanisms measure a focus position (or inclination) at a predetermined measurement point (or measurement area) in an exposure field of the optical projection system and correct the focus position (or inclination) of the wafer by, for example, a servo system based on the measurement result. In this case, because the wafer stands still during the exposure, there no particular inconvenience is caused even if the autofocusing and autoleveling mechanisms have a low response speed.
However, because patterns have become finer and finer for recent semiconductor devices and the like, it is necessary to increase the resolution of optical projection systems. Approaches for enhancing resolution include shortening the wavelength of the exposure light and increasing the numerical aperture of the optical projection system. In either case, if it is desired to secure an exposure field similar to the prior art, it becomes more and more difficult to maintain the imaging performance (distortion, curvature of field, etc.) at a predetermined accuracy over all of the exposure field. Thus, it is desired to use a projection exposure apparatus of a scanning exposure type apparatus such as a so-called slit scanning system or step-and-scan type system (hereinafter called a "scanning exposure type system").
This projection exposure apparatus of a scanning exposure type exposes the pattern of a reticle onto a wafer while scanning the wafer and the reticle in relative synchronization with respect to a radiation region such as a rectangle, an arc, or a plurality of trapezoids having a two-dimensional form (hereinafter called a "slit-shaped illumination region"). Therefore, in a case where a pattern with the same area as that of the stepper type is exposed onto the wafer, the scanning exposure type apparatus can reduce the illumination region of the optical projection system when compared with the stepper type apparatus so that it is possible to obtain improved imaging performance in the illumination region.
In addition, conventionally, the reticle is six inches in size, and the mainstream of projection magnification by the optical projection system is 1/5 times. However, as the area of the circuit pattern for a semiconductor device and the like becomes larger and larger, a reticle six inches in size becomes insufficient under the magnification ratio of 1/5. Thus, it becomes necessary to design a projection exposure apparatus having an optical projection system with changed magnification such as 1/4 times. The scanning exposure type is also advantageous in fulfilling such an increased area of the transferred pattern.
Even such a scanning projection exposure apparatus, however, requires a mechanism which aligns each shot area with the imaging plane during exposure. However, even if, in the scanning exposure type, similar to the stepper type, the focus position (inclination) on the wafer is measured in an actual exposure region and correction is performed based on the result of measurement, there is a disadvantage in that it is difficult to align the actual exposure region with the imaging plane because the wafer is being scanned, and the response of the autofocusing mechanism (or autoleveling mechanism) is at a predetermined value.
A method which is increasingly being provided for detecting a focus position is a so-called read-ahead system which reads in advance the height, inclination, and the like of the wafer at a measurement point in a read-ahead region before the actual exposure region with respect to the scanning direction by taking a response speed into account, and corrects the focus position (or inclination) in the exposure region based on the result obtained by the read-ahead system.
Even when the focus position is detected by such a read-ahead system, if exposure of the wafer is performed for all shot areas in a sequence simply from one end to the other as in the conventional step-and-scan type apparatuses as shown in FIGS. 9(a)-9(c) of Japanese Laid-Open Patent Publication No. 4-196513 (U.S. Ser. No. 068,101 filed by the assignee of this application on May 28, 1993), there arise such problems that, if the shot area contains a periphery of the wafer, the value of the focus position detected in the read-ahead region may vary significantly depending on the scanning direction, and precise position control becomes impossible, that it takes too much time before the inclination of the wafer aligns with that of the imaging plane, or that the amount of correction in the autofocusing mechanism (or autoleveling mechanism) becomes too large, and the actual exposure region cannot follow the imaging plane.
Moreover, for example, if relatively large foreign material such as a scrap of the photoresist is interposed between a wafer and a wafer holder, the height or level of the wafer surface varies. Therefore, there is a problem that if information obtained by the read-ahead system at a region where such a foreign material exists is used as it is, an actual exposure surface cannot be coincide with the imaging plane, since the correction value of the focus position or the inclination becomes too large. Moreover, if the information read ahead at the region where the foreign material exists is used as it is, the focus position or the inclination at a different region greatly deviates from the real value, since the height of the wafer surface greatly and locally varies.
Furthermore, in the step-and-scanning projection exposure apparatus, exposure is performed onto each shot area of a wafer by the scanning exposure type and wafer stepping is performed during exposure onto each shot area. In this case, conventionally, the positioning of the shot area of a wafer to be exposed in a nonscanning direction (a direction perpendicular to the wafer scanning direction) must only have been completed before the exposure onto the shot area to be exposed starts.
As stated above, even in the scanning exposure projection exposure apparatus, it is necessary to continuously carry out autofocusing during scanning exposure. However, in the case of the scanning exposure type, the response speed of the autofocusing mechanism has a predetermined maximum limit if the focusing position is merely measured only in the slit-like exposure region and the height of the wafer is adjusted based on the result of this measurement because the wafer is scanned in a predetermined scanning direction with respect to the projection optical system. There is thus the possibility of the occurrence of follow-up errors between the wafer surface and the imaging surface. Therefore, it is desirable to carry out autofocusing so that follow-up errors will not occur even if the wafer is scanned.
Furthermore, there is an appropriate amount of exposure for the photosensitive materials, such as photoresist, on a wafer. In the scanning exposure type, if the intensity of illumination and the width of the slit exposure region are determined, the scanning speed for obtaining the appropriate amount of exposure will be determined as a predetermined value. Therefore, in carrying out exposure onto a wafer by the scanning exposure type, a predetermined acceleration section (approach run section) is needed before the scanning speed of the wafer reaches a predetermined value. It is desirable that at least autofocusing be conducted before the wafer passes through the acceleration section.
Moreover, in the case of the step-and-scanning projection exposure apparatus, conventionally, there were cases where a wafer moves in the nonscanning direction until immediately before exposure is carried out by the scanning exposure type in a predetermined shot area on a wafer. Thus, there is the possibility of the occurrence of autofocusing follow-up errors if the wafer is moving in the nonscanning direction. Therefore, it is desirable that the arrangement be made so that follow-up errors do not occur.
Moreover, in the step-and-scanning projection exposure apparatus, the speed of wafer movement at the time of stepping is fairly large compared to the speed (scanning speed) of wafer movement at the time of scanning exposure. Therefore, there is a possibility of the occurrence of unstable vibrations in the wafer due to the autofocusing operation at the time of stepping.